1. Field of the Invention
This invention relates to a power transmission apparatus using a continuously variable transmission mechanism for an automobile.
2. Description of the Prior Art
In a torque converter so far often used for an automatic transmission apparatus of an automobile, there occurs substantial slip between its input shaft and output shaft. This type of transmission mechanism has been defective in that its power transmission efficiency is low and fuel efficiency, or mileage, of the automobile is difficult to improve.
Therefore, a continuously variable transmission mechanism having a relatively high power transmission efficiency draws attention for use as a power transmission apparatus for an automobile. It is generally known that power transmission efficiency of the continuously variable transmission mechanism is greatly varied with speed ratio of input and output shafts, rotational speed of input shaft and input shaft torque, high efficiency in low rotational speed and high torque range, while that of a conventional change gear transmission system is almost constant under all conditions (only depending on gear type and the number of engaging teeth). Further, the continuously variable transmission mechanism is more flexible in setting the range of speed ratio as compared with the change gear transmission system, and the speed ratio can be continuously varied within the range. Therefore, the continuously variable transmission mechanism is theoretically advantageous over the change gear transmission system in view of the fuel efficiency and acceleration performance. Especially for the fuel efficiency, in the conventional change gear transmission system, the speed ratio is specified corresponding to a given engine output power and vehicle speed, so the fuel efficiency is inevitably decided. On the other hand, in the continuously variable transmission mechanism, the engine can be operated at a lower speed and a higher torque to obtain the above engine output and vehicle speed by selecting a lower speed ratio, thereby providing maximum fuel efficiency.
However, in a reciprocating engine, there occurs a torque fluctuation caused by the inertia of reciprocating motion of the piston and a fluctuation of pressure in the combustion chamber. Therefore, the crankshaft of the engine rotates being accompanied with the fluctuation in torque and the fluctuation in the rotation speed. Such fluctuation in torque affects the drivability of a vehicle. Especially, when the engine is operated at a low speed, the fluctuation in torque is transmitted to the body and drive shaft of the vehicle, and the lower limit of operation speed of the engine is substantially increased because of vibration due to fluctuation in torque, even if the average drive torque is sufficient to drive the vehicle As a result, there occurs a problem in that, even if the above described continuously variable transmission mechanism is used, the speed ratio must be selected to a substantially large value, resulting in reduced fuel efficiency.
Further, in a conventional belt type continuously variable transmission mechanism in which belt pressing force of the driven pulley is set to a constant value there occurs a problem in that the belt pressing force is excessive for partial load operation resulting in reduced transmission efficiency and shortened life of the belt, and the belt tends to slip at the time of starting the vehicle causing accelerated wearing of the belt.